Like you said, one way of doing this is to create a rounded rectangle 2D path object and using your image as a fill. You can also select your image, copy, select the 2D rounded rectangle and use the paste into options. The 2D path object can also be converted to a selection, then using the convert to object option, the selected part of the image can be converted to an object. Delete the background and you have basically the same thing as the methods above.
If you don't want to do it with path objects, maybe you could press the mask icon and go to the mask mode. Deselect to turn off the selection marquee in the mask mode, apply a gaussian blur, then tweak with the levels sliders to sharpen the transition from white to black. The amount of gaussian blur you apply will determine the extent of roundness in the resulting selection when you go back to normal mode. This method can also be used to round corners of other oddly shaped objects and selections, too.